ABSTRACT The United States is facing a boom in the proportion of its population over the age of 65. This will put enormous strain on the health care system for decades to come unless the impact of age-related diseases can be reduced in the short term. The process of aging is associated with the development of all age-related diseases; telomere length, adult stem cell frequency and function, all decline with age. A variant of the FOXO3 gene at SNP rs2802292 associates with longevity in many human populations. This FOXO3 variant specifically confers a substantially reduced risk of dying of coronary artery disease (>26%), similar in magnitude to common cardiovascular risk factors, such as hypertension, diabetes, smoking. FOXO3 also impacts other age-related diseases, physical and possibly cognitive function. The maintenance of adult hematopoietic- and neural- stem cells is dependent on FOXO3. Despite these associations it is unknown by what cellular and molecular mechanisms FOXO3 influences human longevity and healthy aging. Current studies of potential mechanisms have mostly been limited to cell and animal models; thus, the proposed project will make use of a large, longitudinal clinical cohort to attempt to fill this gap in our understanding. The specific aims proposed to assess the mechanisms which FOXO3 operates via to impact human longevity are: 1 - Test the hypothesis that the most robust protective FOXO3 variant (SNP rs 2802292) affects telomere dynamics in a longitudinal study of a human cohort with several decades of follow up and in cross-sectional studies of male and female offspring; 2 - Test the hypothesis that the protective FOXO3 variant affects blood stem cell numbers and function; and 3: - Test the hypothesis that the longevity-associated FOXO3 variant affects the maintenance of stem cells in the brain and neurological health as a function of age. Telomere attrition with age is variable between individuals, and the rate of attrition is associated with cardiovascular disease risks and mortality. If FOXO3 affects genome stability via an impact on the rate of telomere attrition, or changes in telomere maintenance via telomerase levels or expression, it will be evident in the proposed longitudinal and cross-sectional analyses of telomere dynamics spanning several decades of life. Stem cell populations act as a reserve for replenishment of tissues throughout adulthood, and these populations decline in number and function with age. If the FOXO3 longevity variant contributes to improved maintenance of these cells it should be reflected in the frequency and function of stem cells from the peripheral blood and brain, and with healthier phenotypes of brain aging, such as higher neural stem cell count, reduced accumulation of senescent cells, fewer plaques/tangles, more robust dendrite/spine branching, and/or better cognitive function. The proposed study will perform the first clinical assessment of the relationship between longitudinal telomere dynamics and FOXO3, and the first clinical analysis of the relationship between human FOXO3 longevity variants and changes in stem cell frequency and function with age. The insights gained from greater understanding of the mechanism by which a major human longevity gene affect aging will inform more targeted research that could result in medical innovations that improve quality of life among older adults.